This invention relates to an active filter device for suppressing harmonic current components of a power supply system wherein a compensation object load including harmonic component source and a capacitive load are coupled to a power supply.
FIG. 3 is a circuit diagram showing the organization of an active filter device according to this invention, which is disclosed, for example, in Mitsubishi Denki Giho (Mitsubishi Electric Corporation's Technical Journal) Vol. 62, No. 6, 1988, pp. 15 through 20. Coupled to a power supply 1 is a compensation object load 2, which consists of a harmonic component source 3 such as a cycloconverter and a capacitive load 4 such as a capacitor. A current transformer 5 detects the load current I.sub.l flowing into the compensation object load 2. In response to the detection output of the current transformer 5, an active filter 6 outputs compensation current I.sub.c corresponding to the harmonic current components of the load current I.sub.l. Thus, the harmonic current components contained in the source current I.sub.s are suppressed via the compensation current I.sub.c supplied from the active filter 6.
The above conventional active filter device has the following disadvantage.
When the compensation object load 2 consists solely of a harmonic component source 3, which has a high impedance, the compensation current I.sub.c outputted from the active filter 6 flows into the side of power supply 1 having a low impedance, and the harmonic current components in the power supply 1 can thus be suppressed. However, when the compensation object load 2 of the capacitive load 4 becomes lower than that of the power supply 1 with respect to certain orders of the harmonic, and a portion of the compensation current I.sub.c flows to the side of the compensation object load 2. Thus, certain harmonic current components detected by the current transformer 5 increase. As a result, the current transformer 5 and the active filter 6 undergo a positive feedback, and the operation becomes unstable. Worse still, the harmonic current components of the source current I.sub.s may even increase.